Determining concentration of an analyte in a measured medium plays an important role in many industrial applications, for example, in chemical or pharmaceutical technologies, in foods technology, in biotechnology, as well as also in non-industrial, analytical applications, for example, in environmental measurements technology. Applied for determining ion concentrations frequently in the laboratory, as well as also in industrial process plants, are sensors, which have a sensor element bearing an analyte sensitive component. The analyte sensitive component can be, for example, an analyte sensitive membrane. Thus, for example, the glass membrane of the known pH glass electrode is sensitive to the concentration of H+, respectively H3O+, ions in a measured medium.
Alternatively, the analyte sensitive component can also be a semiconductor element, for instance, a component comprising an EIS structure, such as, for example, an ion sensitive, field effect transistor (ISFET) or a capacitor with an EIS structure, whose capacitance depends on the concentration of the substance to be determined. The acronym “EIS” stands for “Electrolyte Insulator Semiconductor”, i.e. the sensor has a layer structure with, applied on a semiconductor layer or on a semiconductor substrate, at least one insulating layer, which, in measurement operation of the sensor, is in contact with an electrolyte, namely the measured medium. Arising at the interface between the insulating layer and the measured medium is a voltage drop. Through suitable choice of the insulating layer, especially by providing an analyte sensitive coating on, or as a component of, the insulating layer, the sensitivity of the sensor can be set in such a manner that the voltage drop can serve as a measure for the analyte concentration. Thus, for example, the voltage drop at the interface between a tantalum(V) oxide (Ta2O5) layer and an aqueous measured solution depends essentially on the pH value of the measured solution. Through application of other layer structures, EIS sensor elements can be formed, which are sensitive in corresponding manner for other ions. By immobilizing suitable detector structures, which can comprise e.g. enzymes, on the EIS structure, it is also possible by means of such a sensor to measure concentrations of non-ionic substances, e.g. glucose or penicillin.
The already mentioned ion-sensitive field effect transistor has likewise such an EIS structure. The transistor gate of the ISFET is in the case of a pH-sensitive ISFET, for example, formed by a pH sensitive, insulating layer, which can comprise Ta2O5. The charge carrier density in the semiconductor channel between source and drain of the ISFET then depends correspondingly on the pH value of the medium in contact with the gate. German Patent DE 198 57 953 A1, for example, describes a sensor for measuring ion concentrations, respectively the pH value, of a liquid using an ISFET.
Such sensor elements with an analyte sensitive component, especially semiconductor based elements, are frequently embodied in the form of a platelet or chip, for example, as a chip or chip array, with front- or rear, contact elements for electrical contacting of the analyte sensitive component. Sensors with such sensor elements are frequently embodied as rod-shaped measuring probes, which comprise a medium immersible housing, in which the sensor element is so arranged that its analyte sensitive component contacts the measured medium. In such case, the contact elements for electrical contacting of the analyte sensitive component, via which the sensor element is connected with a sensor electronics, and the sensor electronics itself, are arranged protected within the housing. The measuring probe can be connectable via a cable or wirelessly with a superordinated unit, for example, a measurement transmitter or a bus coupler. The superordinated unit can supply the measuring probe with energy, respectively receive, and further-process, measuring signals output from the sensor electronics, or it can output signals to the sensor electronics. Examples of such sensors are disclosed in U.S. Pat. No. 6,117,292, U.S. Pat. No. 6,153,070 or European EP 1 396 718 A1.
Described in European EP 1 396 718 A1 is a sensor having an ISFET as sensor element. By means of a pressure exerting part, the ISFET is pressed at a rear surface facing away from its ion sensitive surface region against an end face of a sensor housing. The pressure exerting part has a central opening, which exposes the ion sensitive surface region of the ISFET, while the source connection and the drain connection of the ISFET are arranged in an interior of the sensor protected from contact by the measured medium. The pressure exerting part is connected with the sensor housing by means of a surrounding ultrasonic weld excluding the medium.
An internal tube extending within the housing divides the housing interior of the sensor housing into a sensor inner space and a sensor intermediate space. The sensor intermediate space serves as reference electrode space, i.e. it contains a reference electrolyte, into which a reference electrode extends. Led through the sensor inner space are connection wires connected with front- or rear contact elements of the ISFET for contacting source and drain of the ISFET. The connection wires serve for connection of the ISFET with a display device of the sensor. The sensor inner space is backfilled from the internal tube to the rear surface of the ISFET with a potting compound, for example, one based on an epoxy adhesive, in order to provide mechanical support for the ISFET.
European EP 1 396 718 A1 shows, by way of example, the previously always followed principle of having a sensor inner space, which serves for the contacting and the guiding of the potential sensing wires of the sensor element, and an electrolyte filled, intermediate space, in which the reference electrode, embodied, as a rule, as a reference electrode of second type, is arranged. Manufacturing these sensors of the state of the art embodied according to this principle requires a number of individual working steps and is, consequently, quite complex and expensive.
Moreover, the sensor housing of such sensors frequently contains, connected with the potential sensing wires, a sensor circuit, frequently an electronic sensor circuit, also referred to as the sensor electronics, which serves for initial processing, especially amplification and, in given cases, digitizing, of the measurement signal. Penetration of moisture into the sensor electronics can lead to failure of the sensor. Consequently, great effort must be applied, in order to assure the sealing of the electrolyte filled, reference electrode space relative to the sensor electronics, which is, as a rule, likewise accommodated in the sensor housing.